OP0113 THE BETA SECRETASE BACE1 DRIVES SYSTEMIC SCLEROSIS FIBROBLASTS ACTIVATION THROUGH Β-CATENIN AND NOTCH SIGNALLING
Authors: C. Wasson, E. Clavane, R. Ross, G. Migneco, C. Antinozzi, J. Brown, L. Di Luigi, C. Mckimmie, F. Del Galdo, P. Meakin
Journal: Annals of the Rheumatic Diseases (2023)

<h3>Background</h3> The beta-amyloid precursor protein cleaving enzyme 1 (BACE1) is well known for its role in the development of Alzheimer's disease via the generation of β-amyloid. Recent publications, including our own, have demonstrated a role for this enzyme in other chronic inflammatory diseases, including type 2 diabetes and cardiovascular disease. However, to date there has been no studies looking into the role of BACE1 in the autoimmune condition Systemic Sclerosis (SSc). <h3>Objectives</h3> The aim of this study was to assess the expression profile of BACE1 in SSc patient samples and investigate the effects of BACE1 inhibitors and siRNA on SSc fibroblast activation. <h3>Methods</h3> Patient fibroblasts were obtained from full thickness forearm skin biopsies from healthy and early diffuse SSc patients. BACE1 was inhibited with 2 specific small molecule inhibitors and siRNA specific to BACE1. Morphogen signalling was activated with recombinant TGF-β, Wnt-3a or the smoothened agonist SAG. A xenotransplant bleomycin mouse model using patient pDC was used to interrogate <i>in vivo</i> expression of BACE1 in fibrosis. <h3>Results</h3> Here we show that BACE1 protein levels are elevated in SSc patient skin biopsies. In particular BACE1 was increased in the fibroblasts and endothelial cells of the SSc skin. BACE1 was elevated in isolated dermal fibroblasts grown in culture (2.3 fold increase, N=4). BACE1 protein levels were elevated in the bleomycin skin fibrosis model. Interestingly BACE1 mRNA levels were unaffected in cultured SSc fibroblasts, suggesting a post-translational modification led to the elevated protein levels. Inhibition of BACE1 with small molecule inhibitors (that have been proven safe in phase 1 clinical trials for Alzheimer's) or siRNA blocked pro-fibrotic gene (alpha SMA, Collagen Type 1 and CTGF) expression in SSc fibroblasts. In addition overexpression of BACE1 in healthy fibroblasts resulted in myofibroblast activation (2-fold increase in alpha SMA protein expression). Interestingly overexpression of a BACE1 mutant construct which disrupts the secretase activity of the protein, was unable to induce fibroblast activation. Disruption of BACE1 (with both the inhibitors and siRNA) blocked morphogen mediated fibroblasts activation. The BACE1 inhibitors and siRNA blocked TGF-β, Wnt-3a and Hedgehog mediated alpha-SMA expression in healthy fibroblasts. Furthermore, we show that BACE1 regulation of dermal fibroblast activation was dependent on the β-catenin and Notch signalling pathways. BACE1 ability to regulate non-canonical Wnt receptors led to elevated β-catenin expression which in turn activated the Notch signalling pathway. <h3>Conclusion</h3> This is the first evidence that BACE1 and in particular its secretase activity, plays a role in SSc and fibrosis in general. The ability of BACE1 to regulate SSc fibroblast activation reveals an exciting new therapeutic target in SSc. Several BACE1 inhibitors have been shown to be safe in phase 1 clinical trials for Alzheimer's disease. Future work includes investigating the role of BACE1 in vascular/endothelial cell dysfunction in SSc. <h3>REFERENCES:</h3> NIL. <h3>Acknowledgements:</h3> NIL. <h3>Disclosure of Interests</h3> None Declared.